Doran's Ring, like any other Doran's item, is very gold efficient because the purpose of Doran's items is to offer a huge amount of early stats pretty much just for the laning phase, without any further scaling. Making that 5 bonus damage unique and named, alongside that same effect on Doran's Shield, would allow just one Doran's item to provide it without it being able to stack, therefore bringing the passive back for mages in a more explicit manner without necessarily affecting their overall build decisions. If the worry is that mages would only start with Doran's Ring, then making it a unique named passive on Dark Seal as well could allow mages to have that bonus integrated into whichever starting item they choose.
On top of that, I would personally support cleaning up mage auto animations, because I don't think any champion really deserves to have an unresponsive autoattack, but I would not support AD or AS buffs, because that would increase those mages' harass potential against their laning opponent. If that is the specific intent, then sure, though that would carry some much larger balance considerations, but the advantage to the 5 bonus damage against minions was that it specifically did not affect poking, trading or harassing other champions. Even making auto animations cleaner is going to improve the harass potential of mages that receive that QoL buff. Because of this, while I wouldn't discount AD/AS buffs entirely, I'd definitely put them on a lesser priority than adding a bonus damage effect on DRing/Dark Seal, and improving mage autoattack animations.